From the Book

Section 3: ISDN (8 Points)

Ensure that VLAN3 and R4 Lo0 are accessible from R1 and beyond should
the Frame Relay network fail either physically or logically. If VLAN3 and R4 Lo0
networks are restored while the ISDN line is active, ensure that traffic is
routed over the Frame Relay network to these destinations immediately.

As no static routes are permitted and backing up the Frame Relay interface
will not help as this only works if the Frame Relay interface is physically
down, the only option will be to use the dialer-watch feature. Both
networks must be down before the router dials out so VLAN3 and R4 Lo0 should be
added to a dialer watch-list and corresponding dialer watch-group
number under the BRI interface on R1. R1 is used to dial out as the question
states that the two networks should be accessible from R1. You are also later
advised that only R1 should dial into R4.

You should notice that when you fail the Frame Relay network to test this
that after the ISDN is activated and the Frame Relay network is then restored
that the routing table on R1 shows identical hop counts for all remote networks
via R4 over both the Frame Relay and ISDN line as shown in Example 1-23.

This condition can keep the ISDN line from ever deactivating as the ISDN
network can now be used as a valid means to transport data to the RIP advertised
remote networks, you should also notice that the question requires that the
Frame Relay routes should be used "immediately" when restored and, at
this point, routers R1 and R4 can choose between Frame Relay and ISDN.

RIP obviously does not take into account the bandwidth of available routes.
You, therefore, need to make the ISDN routes less desirable and add additional
hop count to RIP using an offset-list on R4 and R1 out over the ISDN
line (inbound over both routers will also be acceptable). This ensures when the
Frame Relay is restored and for the period where both Frame Relay and ISDN lines
are active and receiving RIP routes that the hop count is more favorable over
Frame Relay because of the additional hop count incurred over ISDN after the
offset-list is applied.

The ISDN line can not be used to route traffic while a higher-speed Frame
Relay connection is available as shown in Example 1-25. Dialer-watch does
not require interesting traffic to trigger the dial so the dialer-list
should be an implicit deny of any IP traffic; otherwise, any traffic will
potentially keep the line up after initiated. It is better practice and shows a
better understanding of the dialer-watch process to, therefore, have the
following dialer-list on R1; dialer-list 10 protocol ip deny. You
will find with this strict policing of the interesting traffic, your ISDN line
will stay down when the networks are restored over the Frame Relay. If you have
configured this question correctly as in Example 1-24, you have scored 5 points;
if you have used a dialer-list that denies RIP and the line stays down,
you have only scored 3 points. Test your scenario thoroughly if you have first
denied RIP then allowed all other IP traffic and also not applied the
offset-list; you could find that with two routes in the routing table
with identical metrics that traffic, such as BGP, will toggle between the two
routes and keep the line up constantly. In addition, other IP traffic could be
classed as interesting and keep the line up.

Example 1-23 R1 Routing Table Pre Offset-List with the ISDN
Line Active After the Frame Relay Network Has Been Restored

The routing table output is taken after a Frame Relay failure is
restored and the ISDN line is still active. The shading shows you the two
available routes with the identical hop count on R1 before the offset-list
is applied.

Example 1-24 Increasing the Hop Count Out of R4 and R1 ISDN
Configuration

offset-list 0 will apply the chosen additional hop count (2) to all
networks being advertised from R4 and R1 out of their interfaces BRI0/0. A
similar configuration could be placed on each BRI0/0 but inbound.

Example 1-25 R1 Routing Table Post Offset-List with the ISDN
Line Active After the Frame Relay Network Has Been Restored

The routing table output is taken after a Frame Relay failure is restored and
the ISDN line is still active. This shows that the ISDN routes are no longer
entered into the routing table on R1 because of the increased hop count over
this environment. The routing table on R4 will act in exactly the same
manner.

Configure R1 so that if half of the ISDN traffic to R4 is of an
unacceptable quality, the line is automatically disconnected.

Configure ppp quality 50 under both R1 and R4 BRI0/0 interfaces, the
figure (percentage) is for both incoming and outgoing directions on the
interface, PPP will drop the line if the quality falls below 50 percent and
initiate a timer before re-establishing the link. If you have configured this
correctly, you have scored 1 point.

Allow only R1 to dial into R4. Do not use any PPP feature in your
solution.

The question is not seeking configuration of CHAP on both routers as any
router configured with the correct CHAP password could emulate R1 and gain
access to R4. It is, therefore, required to configure R4 with isdn caller
1111 if using legacy DDR or dialer-caller 1111 if using dialer
profiles to ensure that only R1, which is connected to the ISDN number 1111, can
actually gain access by having R4 check the CLI before answering. You may have
automatically assumed this must require CHAP but there is not sufficient detail
in the question to suggest that CHAP or PAP is required. These are both also PPP
features so it is disallowed anyway. If you have configured this correctly, you
have scored 2 points.

NOTE

Your ISDN line or simulator must support CLI to test this feature.

Do not allow the ISDN to flap if the Frame Relay network goes up and
down; only allow the line to be dropped if the Frame Relay connectivity is
deemed to be reliable for 90 seconds.

By default, the ISDN line will be dropped when dialer-watch again has
visibility if the networks listed in the dialer watch-list. To ensure the line
remains active for 90 seconds the command dialer watch-disable 90 should
be added to the BRI0/0 interface of R1. If you have configured this correctly,
you have scored 1 point.

Example 1-26 and Example1-27 show the full final ISDN and relevant RIP
configuration required for the ISDN backup on R1 and R4, using a mix of legacy
and dialer profile commands.